A new study of a dozen NASA astronauts has found that exposure to microgravity causes hearts to become more spherical in shape — a change that could lead to serious cardiac problems while in space and back on Earth.

Thomas' team had astronauts take images of their hearts using ultrasound machines installed on the International Space Station. Results showed that their hearts became more spherical by a factor of 9.4% when in microgravity —a transformation consistent with mathematical models. The change was temporary, with the heart returning to its normal, elongated shape shortly after returning to Earth. The researchers say the spherical shape could mean that the heart is performing less efficiently, though the long-term health effects are still not known.

This is frustrating news given the potential for an 18-month mission to Mars. The researchers say that knowing the amount and type of exercise astronauts need to perform to keep the heart healthy is going to be very important to guarantee their safety on such a long journey.

That said, there's only so much that exercise can do. Clearly, we need to come up with some better solutions; spaceflight is already known to cause orthostatic hypotension (where the body experiences a sudden drop in blood pressure when standing up) and arrhythmias.